How can I trust cheap eyeglasses?

[cheap glasses]

I need to buy a pair of prescription sunglasses in aviator style, as it is summer now, the sun is really shining, but you know that nowadays, though there are so many kinds of eyeglasses frames on the market, the price is uaually really expensive, and I know online sites can provide a much cheap price, some even less than $20, but how can I trust them?